The present invention relates to a speed reduction unit.
Speed reduction units are known which comprise an input shaft, an output shaft, a first gear keyed on the said output shaft and a second gear connected to the said input shaft, and in which a 360.degree. rotation of the input shaft produces a rotation of the output shaft corresponding to an advancement of the said first gear by one or more teeth.
Generally, such known reduction units are of the type with planocentric gears in which the said first gear is an internal gear having n teeth, and the second gear is a gearwheel having n-x teeth and a diameter smaller than that of the said internal gear so that it can be accomodated inside the latter. Moreover, the said gearwheel is rotatably coupled with a central disc which is integral with the input shaft, but is positioned eccentrically with respect to it.
In use, because of the presence of the said eccentric disc, to a rotation of the input shaft does not correspond any rotation of the said gearwheel, but a transversal movement of it, owing to which the teeth of this gearwheel successively mesh with the teeth of the said internal gear. If, as said before, the number of the teeth of the internal gear is n and that of the gearwheel is n-x; to a complete revolution of the input shaft in one direction corresponds a rotation of the output shaft in the opposite direction, equal to a number x (in a border-line event a single tooth) of the internal gear.
It clearly results from the foregoing that to obtain the successive meshing of the teeth of the gearwheel with those of the internal gear, the manufacturing tolerances of the single elements must necessarily be relatively close; accordingly, the manufacturing costs are relatively high and such as to inhibit the use of such reduction units in many cases in which their use would be technically advisable.